Recently, as the back light of an optical instrument such as a liquid crystal projector or an overhead projector, a projection light source that uses a high-pressure mercury vapor discharge lamp, whose continuous emission in the visible range is increased by increasing the vapor pressure of mercury during lighting to the range of 20 to 35 MPa, combined with a reflecting mirror is known (see Japanese Patent Application (Tokkai Hei) No. 2-148561).
The first requirement for the back light used for a liquid crystal projector or the like is a high luminance. For example, in a high-pressure mercury vapor discharge lamp, the luminance requirement can be satisfied by setting the load input per the unit length of the discharge arc (X/L) to 88 or more, in which X (W) is the lamp power during lighting and L (mm) is the distance between the electrodes. In such a lamp, the lighting life is about 4,000 hours at most. Recently, however, a longer life, that is, a life of 4,000 hours or more, is desired.
In conventional high-pressure mercury vapor discharge lamps, as X is higher and L is shorter, the load input per the unit length of the discharge arc (X/L) increases, in which X (W) is the lamp power during lighting and L (mm) is the distance between the electrodes. Thus, the arc temperature increases. For example, when the lamp is designed with a X/L of 88 or more, the tube wall temperature of the arc tube increases during the lighting life because the arc temperature increases excessively. Also, the arc tube gradually expands during the lighting life because the lighting operation pressure is as high as 10 to 35 MPa. Thus, difficulties are caused. For example, the optical characteristics of the lamp change, and the lamp is damaged by the expansion of the arc tube. Therefore, it is difficult to achieve the life of 4,000 hours or more.